Method of abating NOx and preparation of catalytic material therefor

ABSTRACT

A method for reducing gaseous nitrogen oxides present in a gas stream by reaction with a reductant species is practiced by flowing the gas stream under lean NO X  -reducing conditions in contact with a catalytic material containing a catalytically effective amount of a catalytic species, e.g., a platinum group metal, and a reductant storage material, e.g., a zeolite, effective for storing reductant species for reaction with NO X , and providing an intermittent supply of the reductant to the gas stream. The catalytic material may be prepared in any manner, but one method is to incorporate a catalytically effective amount of the platinum into a template-bearing molecular sieve material, preferably ZSM-5, to hinder the platinum from being incorporated into the pores of the molecular sieve material, and then calcining the molecular sieve material, whereby the template is removed from the molecular sieve material after the platinum is incorporated therein. Another method is to add a blocking agent to the molecular sieve material, then incorporate the platinum therein, and then calcine the material to remove the blocking agent. The catalytic material may contain less than about two percent by weight of zeolite plus platinum, e.g., less than about 0.5%, or from about 0.025% to 0.1% or 0.2% platinum.

This application is a divisional of copending application Ser. No.08/430,065 filed Apr. 27. 1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to catalytic materials for the treatment of gasstreams, and in particular to catalytic materials effective for thereduction of nitrogen oxides (NO_(X)). Nitrogen oxides are well-knownnoxious by-products of the high temperature combustion of hydrocarbonfuels by internal combustion engines. The release of these oxides asexhaust emissions has caused environmental concerns leading togovernmental regulations that compel motor vehicle manufacturers toabate the emission of NO_(X). Efforts towards compliance with suchregulations are complicated by the fact that the exhaust gas streamscontaining NO_(X) typically also contain other pollutants which aretypically abated through oxidation reactions, whereas NO_(X) abatementis typically carried out as a reduction reaction in which it is desiredto reduce the NO_(X) to nitrogen while minimizing the formation of N₂ Oand sulfates. Conventional three-way catalysts, which are known fortheir ability to abate hydrocarbons, carbon monoxide and NO_(X) understoichiometric conditions, are not effective for NO_(X) reduction inlean environments, i.e., in environments in which oxygen is present inexcess of the stoichiometric quantity required to oxidize thehydrocarbons, partially burned hydrocarbons and carbon monoxide in thegas stream.

2. Related Art

One known method for the reduction of NO_(X) from lean emissions is toflow the exhaust gas containing the NO_(X) in contact with a zeolitecatalytic material comprising, for example, ZSM-5, which has beenion-exchanged with copper. Such catalyst was found to reduce NO_(X)under lean conditions using unburned hydrocarbons in the exhaust gas asreductants, and was found to be effective at temperatures from about350° C. to 550° C. However, such catalysts are often lacking indurability, in that catalytic performance usually decreasessignificantly after exposure of the catalyst to high temperature steamand/or SO₂.

Catalysts based on platinum-containing materials have also been found toabate NO_(X) in lean environments, but such catalysts tend to produceexcessive quantities of N₂ O, and also to oxidize SO₂, which is presentin the exhaust as a result of the oxidation of the sulfur component offuels, to SO₃. Both products are undesirable; N₂ O fosters anenvironmental greenhouse effect while SO₃ contributes to the formationof particulate matter in exhaust emissions by reacting to form sulfateswhich add to the particulate mass. Accordingly, there is a need for acatalyst that reduces NO_(X) to N₂ while producing only limitedquantities of N₂ O and SO₃.

Japanese Patent H1-135541 (1989) of Toyota Jidosha K.K. et al disclosesa catalyst for reducing NO_(X) in lean car exhaust comprising zeolitesthat contain one or more platinum group metals, including ruthenium, byion-exchange into the zeolite. In the exemplified embodiments, 100 gramsof a washcoat comprising 150 parts zeolite and 40 parts of a mixture ofalumina sol and silica sol having a 50:50 Al:Si ratio is coated onto acarrier. The following amounts of platinum group metals are thenincorporated into the zeolites: in Examples 1 and 2, 1.0 gram platinum(1.27% by weight of zeolite plus platinum) and 0.2 grams rhodium (0.25%by weight zeolite plus rhodium); Example 3, 1.0 gram palladium; Example4, 1.2 grams ruthenium (1.5% by weight zeolite plus ruthenium); Example5, 1.2 grams iridium. Comparative examples were prepared withoutzeolite.

U.S. Pat. No. 5,330,732 to Ishibashi et al, dated Jul. 19, 1994, teachesthat one or more of platinum, palladium and rhodium can be loaded ontozeolites "by an ion exchange and by an immersion" (column 3, lines 11-17and 22-30) to produce NO_(X) -reducing catalysts. Durability is improvedby using at least 1.3 parts platinum. The platinum group metals are usedseparately in the following amounts per 100 parts by weight ("parts") ofzeolite; platinum, 1.3 parts or more; palladium, 0.8 parts or more; orrhodium, 0.7 parts or more. In terms of the weight of the metals as apercent of the combined weight of the metal plus zeolite, thesequantities correspond to 1.28% platinum, 0.79% palladium, and 0.7%rhodium. The graphs of FIGS. 1-6 of Ishibashi et al plot NO_(X)conversion against platinum group metal loadings and show data pointswhich appear to start at about 0.2 parts of platinum group metal, about0.2%. However, the data show that the claimed amount of at least about1.28% of platinum must be used to attain satisfactory NO_(X) conversion.Preferred zeolites have a pore size of 5 to 10 Angstroms.

U.S. Pat. No. 4,206,087 to Keith et al, dated Jun. 3, 1980, teaches thata NO_(X) -reducing catalyst may comprise 0.01 to 4 weight percent,preferably 0.03 to 1 weight percent platinum group metals dispersed onan inorganic support material that may comprise an alumino-silicate.

U.S. Pat. No. 5,041,272 to Tamura et al, dated Aug. 20, 1991, teachesthat hydrogen form zeolites are catalytically effective NO_(X) -reducingcatalyst materials at 400° C. (see Example 1, column 3).

SUMMARY OF THE INVENTION

The present invention provides a method for reducing gaseous nitrogenoxides present in a gas stream by reaction with reductant species. Themethod comprises flowing the gas stream under lean NO_(X) -reducingconditions in contact with a catalytic material comprising acatalytically effective amount of a catalyst species incorporated into areductant storage material, and providing an intermittent supply ofreductant to the gas stream.

According to one aspect of the invention, the catalytic species maycomprise a platinum group metal and the reductant storage material maycomprise a molecular sieve material. The molecular sieve material may beselected from the group consisting of ZSM-5, Y-zeolite, mordenite,Beta-zeolite, omega-zeolite, rho-zeolite, borosilicates, and ironsilicates. It is preferred that the molecular sieve material has anaverage pore diameter of not greater than about 10 Angstroms. Thus,ZSM-5 is a preferred reductant storage material.

According to another aspect of the invention, the step of providing anintermittent supply of reductant species may comprise pulsinghydrocarbons into the gas stream in amounts that yield, during thehydrocarbon-on modes established thereby, a ratio of carbon atoms toNO_(X) molecules in the gas stream in the range of from about 0.5:1 to20:1.

In particular embodiments, the catalytic material comprises less thanabout 2.0% platinum by weight of zeolite plus platinum, typically lessthan about 0.5% platinum, e.g., from about 0.025% to 0.2%.

The invention also relates to a catalytic material useful for thereduction of NO_(X). The material comprises a porous molecular sievematerial having a catalytic species incorporated therein wherein thecatalytic species is concentrated at the inlets of the molecular sievematerial pores or near the molecular sieve material surface. Theinvention also relates to two methods for making such catalyticmaterials. The first method comprises incorporating a catalyticallyeffective amount of a catalytic species into a template-bearingmolecular sieve material, and then calcining the molecular sievematerial, whereby the template is removed from the molecular sievematerial after the catalytic species is incorporated therein. The secondmethod comprises depositing a blocking agent onto a molecular sievematerial, incorporating a catalytically effective amount of a catalyticspecies into the molecular sieve material having the blocking agentthereon, and then calcining the molecular sieve material, whereby theblocking agent is removed from the molecular sieve material after thecatalytic species is incorporated therein.

Other aspects of the present invention are disclosed in the followingdescription.

As used here and in the claims, the term "platinum group metal" meansand includes platinum, palladium, rhodium, iridium, ruthenium andosmium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-16 are plots showing NO_(X) conversion and N₂ O formation ratesfor the various catalytic materials described in the Examples.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF

The present invention pertains to methods and materials for attainingimproved reduction of oxides of nitrogen (NO_(X)) with hydrocarbonreductants in a gas stream produced under lean conditions, i.e., in agas stream containing oxygen in excess of the stoichiometric quantityrequired to oxidize the hydrocarbons, partially burned hydrocarbons andcarbon monoxide in the gas stream. The NO_(X) reduction reaction iscatalyzed by a catalytic material generally comprising a catalyticspecies, e.g., one or more platinum group metals, incorporated into areductant storage material, i.e., a molecular sieve material, preferablyan acid-form zeolite, which can store reductant from the gas streamand/or catalytically produce and store derivatives therefrom that arecapable of reducing NO_(X) in the gas stream. The derivatives may bemore reactive with NO_(X) than the reductants in the gas stream. Thereductants in the gas stream and the reductant derivatives formed on themolecular sieve material are referred to collectively herein as"reductant species".

A method aspect of the invention involves providing gaseous or liquidreductant in the gas stream intermittently rather than continuously. Forexample, a hydrocarbon reductant such as propylene or diesel fuel may beprovided to the gas stream in alternating intervals, e.g., it may beadded for a designated time interval followed by an interval without theaddition of the hydrocarbon reductant. Stated differently, the reductantmay be pulsed "on" (establishing a "hydrocarbon-on" mode), then "off"(establishing a "hydrocarbon-off" mode) and so on. The hydrocarbon-onand hydrocarbon-off modes may be, but need not be, of equal duration.The intermittent addition of reductant such as hydrocarbons to theNO_(X) -containing gas stream is sometimes referred to herein as"pulsing". The hydrocarbon reductant may be pulsed on by injectingreductant into the gas stream or by appropriately adjusting thefuel/oxygen ratio of the combustion mixture from which the gas stream isderived. In hydrocarbon-on modes, hydrocarbon reductant is provided tothe lean gas stream in an amount that provides a ratio of carbon atomsto NO_(X) molecules in the range of about 0.5:1 to 20:1, e.g., fromabout 1:1 to 8:1. During the hydrocarbon-off modes, reductant is presentin the gas stream in smaller amounts, or not at all, to create areductant deficit in the gas stream that allows reductant species storedin the reductant storage material to be consumed for the reduction ofNO_(X).

Without wishing to be bound by any particular theory, the improvedcatalytic activity attained by pulsing reductant in the gas stream isbelieved to be due to the storage by the catalytic material of reductantspecies while the reductant is pulsed on. The stored reductant speciesare then available for NO_(X) reduction when the reductant in the gasstream is pulsed off. In addition, by allowing stored reductant speciesto react with NO_(X), the catalytic material is "cleaned", exposingcatalytic sites that would otherwise be made inaccessible to NO_(X)and/or gaseous reactant species due to the accumulation of storedreductant species thereon. Further, allowing stored reductant species toreact with NO_(X) diminishes the reductant requirement for the reductionprocess. Thus, intermittently supplying the reductant enhances catalyticactivity of the catalytic material and reduces the quantity of reductantconsumed in the NO_(X) reduction process, i.e., it lowers the reductantcost of the reduction process, relative to a constant supply process. Asis illustratated in the Examples below, the advantage of reductantpulsing is, in part, temperature-dependent; for a given catalyticmaterial being used under given process conditions, there is atemperature above which the advantages of reductant pulsing are nolonger obtained. This is believed to be due to the inability of thestorage material to effectively store reductant species, which may thenbe lost by desorption and/or oxidation at or above that temperature evenwhen the reductant is pulsed on. The upper temperature limit will varydepending on the particular catalytic material, the particular reductantspecies added to the gas stream and the operating conditions under whichNO_(X) reduction is carried out. For a ZSM-5-based catalytic materialcontaining 0.1% to 1.0% platinum subjected to hydrocarbon pulsing withpropylene as the reductant in a gas stream flowing at 25,000/hr, theupper temperature limit for reductant storage falls in the range ofabout 250° C.-300° C.

Molecular sieve materials such as zeolites, borosilcates and ironsilicates are all believed to be suitable storage materials for thepreparation of catalytic material in accordance with the invention.Preferably, the molecular sieve material has an average pore size in therange of about 4 to 10 Angstroms, since this is believed to limit thesize of platinum group metal crystallites that can form during use ofthe catalyst. Since the agglomeration of the platinum group metal intolarge crystallites diminishes catalytic activity, the small average poresize is believed to improve catalyst durability.

While the reductant pulsing method described herein can be practicedusing conventional molecular sieve-based catalytic materials, there isan aspect of the present invention that relates to a method of preparingnovel molecular sieve-based catalytic materials. According to thismethod, a molecular sieve material made with the use of an organictemplate such as a tetraalkylammonium hydroxide is not calcined betweenthe time it is formed and the time the catalytic species, e.g.,platinum, is incorporated therein, as would conventionally be done toremove the organic template. Thus, the platinum (or other catalyticspecies) is incorporated into a template-bearing molecular sievematerial. Without wishing to be bound by any particular theory, it isbelieved that having the template within the molecular sieve materialwhen the platinum is incorporated therein may hinder the platinum frombeing deposited within the molecular sieve material pores, and thusguide the platinum to sites that are more accessible to NO_(X) moleculesthan the platinum would occupy had the molecular sieve material beenpre-calcined. The sites where the platinum is incorporated into atemplate-bearing molecular sieve material are believed to beconcentrated about the inlets of the pores or near the surface of themolecular sieve material. In contrast, platinum would be incorporatedthroughout a non-template-bearing molecular sieve material, including inthe interior of the pores, which are less accessible to NO_(X) andreductant in the gas stream than the pore inlets or the molecular sievematerial surface. Further, the calcination of the molecular sievematerial after the incorporation therein of the platinum group metal isbelieved to provide the finished catalytic material with morecatalytically active acid sites than are formed in molecular sievematerials that are pre-calcined.

Another way that platinum group metal can be guided to more strategicsites is to add a blocking agent to the molecular sieve material beforeincorporating the catalytic species therein. The blocking agent istypically a bulky organo-amine such as a tetraalkylammonium hydroxidethat can obstruct the pores of the molecular sieve material while thecatalytic species, e.g., platinum, is incorporated therein and that canbe removed thereafter by calcination. The blocking agent can be appliedto a molecular sieve material by wetting the molecular sieve materialwith a solution containing the blocking agent and then drying themolecular sieve material. The platinum (or other catalytic species) isthen incorporated into the molecular sieve material and concentrated onthe molecular sieve material surface and at its pore inlets. Subsequentcalcination burns off the blocking agent while fixing the platinum tothe molecular sieve material.

The practice of the reductant pulsing method of the present invention isnot limited, however, to the use of catalytic material prepared usingtemplate- or blocking agent-bearing molecular sieve material. Rather,reductant pulsing can be practiced with molecular sieve materialbasedcatalytic materials prepared in a conventional manner, i.e., usingmolecular sieve materials that are calcined to remove the organictemplate before the catalytic species are incorporated therein.

Yet another aspect of the invention relates to novel platinum-containingzeolite materials that are effective for NO_(X) reduction, the materialscomprising less than about 0.5% platinum by weight. As will bedemonstrated below, zeolite materials comprising as little as 0.025%platinum, more typically 0.025% to 0.1% or to 0.2% platinun by weight ofplatinum plus zeolite, are catalytically effective for such NO_(X)reduction.

A platinum group metal catalytic species can be incorporated in thereductant storage material in any convenient manner. In the case ofzeolite reductant storage material, the platinum group metal, e.g.,platinum, can be incorporated therein by impregnation or by ionexchange. Impregnation is typically accomplished by wetting the zeolitematerial with a solution containing a platinum salt dissolved therein,and precipitating the platinum onto the zeolite. For ion exchange, thezeolite material is soaked or flushed with a solution of one or moresuitable platinum group metal compounds for a period of time andotherwise under conditions to cause the platinum group metal cations todisplace other cations (such as Na⁺, NH₄ ⁺, H⁺, etc.) present in thezeolite material, and thus become incorporated into the zeolitematerial.

A catalytic material employed for gas phase NO_(X) reduction inaccordance with the present invention is rendered in a form in which thegas stream can come into contact with the material under lean NO_(X)-reducing conditions. Typically, this involves depositing the catalyticmaterial as a coating on a carrier which has a physical structure thatallows the gas stream to flow therethrough in contact with the catalyticmaterial at a temperature sufficient to support the reduction reaction,e.g., at least about 150° C. The preferred carriers compriseceramic-like materials such as cordierite, α-alumina, mullite, and thelike, while others may comprise refractory metals such as stainlesssteel. One typical kind of carrier comprises a body of cylindricalconfiguration (which in cross section may be circular, oval orpolygonal) having two end faces and a plurality of fine, substantiallyparallel gas flow passages extending therethrough and connecting the endfaces of the carrier to provide a "flow through" type of carrier. Suchcarriers may contain up to about 700 or more flow channels ("cells") persquare inch of cross-sectional flow area, although carriers having farfewer cells per square inch ("cpsi") may also be useful. For example,typical carriers have from about 200 to 400 cpsi.

The catalytic material can be deposited on the carrier by disposing thematerial in an aqueous slurry and applying the slurry as a washcoat ontothe carrier. A binder material such as silica sol or alumina sol may beadded to the slurry to enhance the adhesion of the catalytic material tothe carrier surface.

The superior catalytic performance attainable by the hydrocarbon pulsingmethod of the present invention is illustrated in Example 1. Theunexpected advantage of incorporating catalytic material into anuncalcined, template-bearing molecular sieve material, rather than intoa pre-calcined molecular sieve material, is illustrated in Example 2.The effectiveness of a zeolite-based catalytic material comprising aslittle as 0.025% platinum is shown in Example 3. Example 4 illustratesthat the surprising advantage of the hydrocarbon pulsing technique ofthe present invention may be attained even after a conventional steadystate process has led to a decrease in catalytic activity, and Example 5illustrates that zeolite-based catalytic material having a low platinumloading exhibits better selectivity for NO_(X) reduction over theunwanted production of N₂ O and SO₃ relative to high platinum loading,especially at low temperatures. Example 6 shows that the improvedselectivity shown in Example 5 is also attainable in the reductantpulsing method of the invention. The molecular sieve materials used inExamples 1 through 6 are all acid-form zeolites having a silica:aluminaratio in the range of 40:1 to 50:1. Example 7 shows that catalyticmaterials comprising acid form zeolites exhibit unexpectedly superioractivity relative to sodium form zeolites.

EXAMPLE 1

A. PREPARATION OF CATALYST MEMBER E-1

A 0.2% platinum-ZSM-5 catalytic material in accordance with the presentinvention was prepared by dissolving 0.4 grams of Pt(NH₃)₄ Cl₂ in 600 mlwater. The pH of the solution was increased from 5.4 to 10.4 by theaddition of an ammonium hydroxide solution. One hundred grams ofuncalcined, i.e., template-bearing ZSM-5 material, was added to thesolution, which was stirred for about five hours at about 45° C. toexchange the platinum cations into the ZSM-5. The solution was thenfiltered and washed with one liter of water and disposed in an aqueousslurry having 33 percent solids content. The slurry was coated onto a400 cpsi cylindrical honeycomb carrier measuring 1.5 inches in diameterand 3 inches in length, at a loading of about 2 grams per cubic inch.The coated carrier was dried at 100° C. and calcined at 550° C. for twohours, which is sufficient to remove the template from the zeolite. Theresulting catalyst member is designated E-1.

B. TEST PROCEDURES A AND B

Catalyst member E-1 was tested by heating the catalyst member to about100° C. in air. A feed stream comprising 250 ppm NO, 333 ppm propylene(equivalent to 1000 ppm C₁), 10 percent H₂ O, 10 percent O₂, 50 ppm SO₂and balance nitrogen, giving a C₁ :NO_(X) ratio of 4:1, was flowedthrough the catalyst member at a space velocity of 25,000/hr, and thetemperature of the gas stream was kept constant at about 200° C. (±5°C.). This steady-state test procedure is referred to herein as TestProcedure A. Catalyst member E-1 was also subjected to a reductantpulsing test using a gas stream like that of Test Procedure A exceptthat the propylene was pulsed on to 333 ppm and then pulsed off foralternating thirty-second intervals. This constant temperature,hydrocarbon pulsing test procedure is referred to herein as TestProcedure B. The rates of catalytic conversion of NO_(X) were monitoredduring both tests and are plotted in FIG. 1. The hydrocarbon pulsingresults shown in FIG. 1 illustrate that while NO_(X) reduction increasedsubstantially during the hydrocarbon-on modes, there was a significantdegree of NO_(X) conversion at 200° C. even during the hydrocarbon-offmodes. Since the hydrocarbons are believed to be the reducing agent forthe NO_(X), the NO_(X) conversion activity during the hydrocarbon-offmode is believed to be due to the reaction with NO_(X) of hydrocarbonsand hydrocarbon derivatives, i.e., reductant species, stored on thezeolite component of the catalytic material.

C. ADDITIONAL TESTS

Catalyst member E-1 was tested again at 250° C., once under the steadystate conditions of Test Procedure A and under the hydrocarbon pulsingconditions of Test Procedure B. The NO_(X) conversion rates are plottedin FIG. 2. The plots show that at 250° C., the catalytic materialperformed better under steady state conditions than under hydrocarbonpulsing conditions. This is believed to be due to the inability of thecatalytic material to store the hydrocarbon reductant at the testtemperature.

D. TEST PROCEDURE C

Test Procedure B was repeated on catalyst member E-1 except that thetemperature of the gas stream started at about 190° C. and was increasedto about 265° C. at a rate of 10° C. per minute. This rising-temperaturehydrocarbon pulsing test procedure is referred to herein as TestProcedure C. The results are plotted in FIG. 3 which shows, as thetemperature approached 250° C., NO_(X) conversion activity in thehydrocarbon-off modes fell to unfavorable rates. This result confirmsthe observation of Example 1 that for the described catalytic materialworking under the previously described conditions, hydrocarbon pulsingyields improved performance at temperatures below about 250° C.

EXAMPLE 2

A. PREPARATION OF CATALYST MEMBER C-1

A 0.2% platinum-ZSM-5 catalytic material was prepared by first calcining1000 grams of ZSM-5 in air in a muffle furnace at 600° C. for two hoursto remove organic templates. The temperature, was increased from ambientin 50° C. intervals over a period of five hours to attain the 600° C.calcination temperature, which was maintained for two hours. Aftercalcination, platinum cations were exchanged into 100 grams of thezeolite material in a manner similar to that described in Example 1. Theresulting catalytic material comprised about 0.2% by weight platinum.The catalytic material was made into a slurry and was coated onto ahoneycomb carrier. The resulting catalyst member is designated C-1.

B. TEST

Catalyst members E-1 and C-1 were both subjected to a test proceduresimilar to Test Procedure A except that the temperature, which startedat about 100° C., was increased at a rate of 10° C. per minute. Thissteady feed stream, increasing temperature test procedure is referred toherein as Test Procedure D. The NO_(X) and hydrocarbon conversion ratesand the N₂ O formation rates were monitored and are plotted in FIG. 4and FIG. 5, respectively. A comparison of FIGS. 4 and 5 show that thecatalytic material of catalyst member E-1, which was prepared fromuncalcined, template-containing ZSM-5, gave surprisingly superiorperformance, including a conversion rate of 50%-60% in the temperaturerange of 200° C.-350° C., relative to catalyst member C-1, which wasprepared from pre-calcined zeolite material from which the template wasremoved prior to the incorporation of platinum and which yielded lessthan 20% NO_(X) conversion in the temperature range 200° C.-350° C.

EXAMPLE 3

A. PREPARATION OF CATALYST MEMBER E-2

A 0.025% Pt-ZSM-5 catalytic material was prepared by adding 0.08 gramsof Pt(NH₃)₄ Cl₂ to 750 ml of deionized water. The pH of this solutionwas adjusted to pH 10.5 using a dilute NH₄ OH solution. To this Ptsolution, 150 grams uncalcined, template-containing ZSM-5 was added. Theresulting slurry was stirred for 3 hours at 50° C. The Pt-ZSM-5catalytic material, which contained 0.025% Pt by weight based on thecombined weight of zeolite plus platinum, was then filtered, washed with500 ml of deionized water and dried overnight on a buchner funnel.

Ninety grams of the dried Pt-ZSM-5 powder was added to 120 grams ofdeionized water and mixed in a blender to produce a washcoat slurry. A400 cpsi cordierite honey-comb was coated with this slurry to give aloading of 1.8 g/in³ of washcoat material after drying at 110° C. andcalcination of 2 hours at 550° C. The resulting catalyst member isdesignated E-2.

B. PREPARATION OF CATALYST MEMBER E-3

A 0.05% Pt-ZSM-5 catalytic material was prepared as described above inExample 3, except that the initial platinum solution was prepared byadding 0.15 grams of Pt(NH₃)₄ Cl₂ to 750 ml deionized water. Theresulting catalyst member is designated E-3.

C. TEST

Catalyst members E-2 and E-3 were tested under steady feed, increasingtemperature conditions according to Test Procedure D, and the resultsare set forth in FIGS. 6 and 7, respectively. These Figures show thatcatalytic materials having as little as 0.025% and 0.05% platinumion-exchanged into the zeolite are effective for NO_(X) reduction at lowtemperature without producing significant quantities of N₂ O or SO₃.

EXAMPLE 4

A. PREPARATION OF CATALYST MEMBER E-4.

A catalytic material comprising 0.1 percent ruthenium made from Ru(NH₃)₄ Cl₂ and 0.2 percent platinum incorporated by ion exchange into ZSM-5zeolite was prepared as generally described above in Example 1. Theresulting catalyst member is designated E-4.

B. TEST

Catalyst member E-4 was subjected to three separate series of tests. Afirst steady-state test similar to Test Procedure A except for thetemperature, was performed. Catalytic activity was observed at 215° C.,270° C. and 315° C. after fifteen minutes TOS. The observed results ofNO_(X) conversion and N₂ O formation are summarized below in TABLE I. Amore complete presentation of the conversion data for the test at 215°C. is provided by FIG. 8.

                  TABLE I                                                         ______________________________________                                        TYPE B TEST                                                                   TEMPERATURE                                                                              % NO.sub.x CONVERSION                                                                        % N.sub.2 O YIELD                                   (° C.) ± 5° C.                                                          AFTER 15 MIN TOS                                                                              AFTER 15 MIN TOS                                   ______________________________________                                        215° C.                                                                           14             4                                                   270° C.                                                                             32              19                                               315° C.                                                                             19              7                                                ______________________________________                                    

After the 15-minute steady state tests, catalyst member E-4 was exposedto 250 ppm NO, 10 percent O₂, 10 percent H₂ O and 50 ppm SO₂ for severalminutes at 215° C. and a space velocity of 25,000/hr for about three tofive minutes to allow the catalyst activity to stabilize. Then,propylene was added intermittently in accordance with Test Procedure B.The results are plotted in FIG. 9, which shows that pulsing yieldssuperior conversion rates relative to the steady state test, even for acatalytic material that has previously been subjected to steady stateconditions.

After the hydrocarbon pulsing test, catalyst member E-4 was heated to270° C. in a gas mixture comprising 10 percent O₂ and 90 percent N₂. Atthat temperature, the catalyst produced 40 ppm CO₂, which is believed tobe the result of the combustion of carbonaceous material stored by thecatalyst member during the previous test. After about five minutes at270° C., the catalyst member was stabilized and the feed stream wassupplemented with 277 ppm NO, 50 ppm SO₂, and 10 percent H₂ O. Upon theintroduction of the NO, CO₂ production increased sharply to about 400ppm, accompanied by about 55 ppm NO_(X) reduction. After about tenminutes NO_(X) reduction fell to about 10 percent, i.e., a removal of 28ppm NO_(X) from the gas stream, and CO₂ production fell to about 200ppm. This result shows that at 270° C. this catalyst is more effectiveat catalyzing the reaction between NO_(X) and hydrocarbons than betweenoxygen and hydrocarbons. This selectivity is advantageous in that thehydrocarbons are consumed efficiently, reducing the supply required tosupport NO_(X) reduction.

After conditions were stabilized at 270° C., hydrocarbon pulsing wasinitiated once again to produce thirty-second hydrocarbon-on intervalsfollowed by thirty-second hydrocarbon-off intervals, for fifteenminutes. The catalyst conversion activity was monitored and the resultsare set forth in FIG. 10. The results are similar to the test at 215° C.in that NO_(X) conversion and N₂ O formation peak at the onset of eachhydrocarbon-on mode and reach a non-zero minimum during thehydrocarbon-off modes. The maximum and minimum NO_(X) conversion ratesand N₂ O formation rates at the 215° C. and 270° C. tests were recordedafter fifteen minutes time on stream and the results are set forth belowin TABLE II.

                  TABLE II                                                        ______________________________________                                        TEMPER-  % NO.sub.x CONVERSION                                                                         % N.sub.2 O YIELD                                    ATURE      AFTER 15 MIN TOS                                                                             AFTER 15 MIN TOS                                    (° C.) ± 5° C.                                                        MAXIMUM   MINIMUM   MAXIMUM MINIMUM                                  ______________________________________                                        215° C.                                                                         52        20        12      5                                        270° C.                                                                           57       26        22      12                                      ______________________________________                                    

The data of TABLE II show, by comparison to those of TABLE I, that theconversion performance of zeolite-based catalytic materials improves,i.e., provides a high maximum NO_(X) conversion rate, when the reductantin the gas stream is "pulsed", i.e., added to the gas streamintermittently, relative to when the reductant level is constant.

EXAMPLE 5

A. PREPARATION OF CATALYST MEMBER E-5

A 0.1% Pt-impregnated ZSM-5 catalytic material was prepared by diluting0.84 grams of an aqueous platinum amine hydroxide solution comprising17.91% platinum by weight in 210 ml of water for impregnation into azeolite storage material. One hundred fifty grams of ZSM-5 was added tothe diluted solution, followed by 7.5 ml of acetic acid. The mixture wasball milled for 16 hours. The resulting slurry was then used to coat acylindrical substrate (400 cpsi) with 1.5" diameter×3.0" length. Thewashcoat loading was about 2 g/in³. The coated substrate was then driedat 100° C. and calcined at 550° C. for 2 hours. The resulting catalystmember is designated E-5.

B. PREPARATION OF CATALYST MEMBER C-2

A comparative 2% Pt on ZSM-5 catalytic material was prepared byimpregnating 200 grams of ZSM-5 with 22.8 grams of 17.91% platinum aminehydroxide solution dissolved in 77.2 grams of deionized water. Aftermixing for one hour, 10 ml of acetic acid was added. Theplatinum-impregnated zeolite material was then dried at 100° C. andcalcined at 550° C. for 2 hours. The finished catalytic materialcontained 2% Pt based on the combined weight of zeolite plus platinum.Eighty-five grams of the calcined catalytic material was used to preparea washcoat slurry with 120 grams of deionized water and 14.9 grams ofNALCO 1056™ (26% silica/4% alumina) binder. The slurry was used to coat1.5 inches×3.0 inches cordierite honeycomb with 2.1 g/in³ of washcoatmeasured after drying at 100° C. and calcining at 550° C. The resultingcatalyst member is designated C-2.

C. TEST

Catalyst member E-5 and comparative catalyst member C-2 were subjectedto the steady feed stream, rising temperature Test Procedure D describedabove. The results are set forth in FIGS. 11 and 12. FIGS. 11 and 12illustrate that catalyst member E-5 exhibited NO_(X) conversion over abroader temperature range than comparative catalyst member C-2. FIG. 11shows that N₂ O production by comparative catalyst C-2 exceeded that ofcatalyst member E-5, particularly at lower temperatures, i.e., around200° C. FIG. 12 shows that comparative catalyst member C-2 beganconverting SO₂ to SO₃ at about 175° C., whereas SO₂ conversion bycatalyst member E-5 did not occur until around 250° C. Thus, FIGS. 11and 12 show that by limiting the quantity of platinum in the catalyticmaterial, NO_(X) conversion is attained over a broader temperature rangeand N₂ O and SO₃ formation are reduced, particularly at lowertemperatures. The conversion and formation activity data of FIGS. 11 and12 for catalyst member E-5 are set forth again in FIG. 13 together withthe observed hydrocarbon conversion rate to illustrate that thecatalytic material was effective to convert hydrocarbons as well asNO_(X).

EXAMPLE 6

Comparative catalyst member C-2 (2% Pt ZSM-5) was subjected tohydrocarbon pulse testing as described in Test Procedure B, at 200° C.The results are set forth in FIG. 14, together with the pulse testresult of catalyst member E-1 (0.2% Pt ZSM-5) from FIG. 1. It is clearthat from FIG. 14 the pulsing results of a catalytic material inaccordance with the present invention are superior, especially duringthe first fifteen minutes of time on stream, relative to that of thecomparative catalytic material. In particular, catalyst member C-2exhibited extremely large variations in conversion rates between thehydrocarbon-on and hydrocarbon-off modes, indicating a lack of catalyticactivity during the hydrocarbon off modes that suggests that thecatalytic material was unable to store reductant species. Further, theconversion rates during the hydrocarbon on modes were lower for catalystmember C-2 than for E-1 for about the first seven minutes of the test.These data illustrate that the advantages of hydrocarbon pulsing is bestrealized with zeolite-based catalytic material having a low platinumloading.

EXAMPLE 7

A. PREPARATION OF CATALYST MEMBER C-3

A 0.2% Pt-Na-ZSM-5 catalytic material was prepared by placing 150 gramsof ZSM-5-type material which is believed to have a silica: alumina ratioof 27 and to be pre-calcined to remove the template, in a solutioncomprising 100 grams of NaNO₃ dissolved in 600 ml water. The mixture wasstirred at room temperature for two hours to exchange sodium cationsinto the zeolite material. The material was then filtered and washedwith one liter of water and dried in air overnight.

A platinum salt solution was prepared by dissolving 0.53 grams ofPt(NH₃)₄ Cl₂ in 250 ml water. One hundred fifty grams of the Na-ZSM-5material was added to the platinum solution, and the mixture was ballmilled and comprised 35% solids by weight. The catalyst material wascoated onto a carrier substrate and was dried at 100° C. and calcined at550° C. for two hours. The resulting catalyst member is designated C-3.

B. PREPARATION OF CATALYST MEMBER E-6

A 0.1% Pt-ZSM-5 material was prepared using the same ZSM-5-type materialused to prepare catalyst member C-3 by dissolving 0.212 grams ofPt(NH₃)₄ Cl₂ in 650 ml water. The pH of the solution was adjusted to10.5 using a solution of NH₄ OH. One hundred grams of the ZSM-5-typematerial was added to the solution and mixed therein for three hours at45° C. The mixture was then filtered, washed with water and coated ontoa honeycomb carrier substrate which was then dried at 100° C. andcalcined at 550° C. for two hours. The resulting catalyst member wasdesignated E-6.

C. TESTS

Catalyst members C-3 and E-6 were each tested in accordance with TestProcedure D described above. The NO_(X), SO₂ and hydrocarbon conversionrates were plotted and are shown in FIGS. 15 and 16, respectively. FIG.15 illustrates that catalyst member C-3 did not exhibit significantNO_(X) conversion until a temperature of at least about 250° C. wasattained. On the other hand, FIG. 16 shows that catalyst member E-6,which was prepared using acid form zeolite, exhibited a low temperatureNO_(X) conversion peak at about 200° C. which was not exhibited bycatalyst member C-3. It is noted that catalyst member E-1 (0.2%Pt-ZSM-5), prepared from acid said form zeolite material also exhibiteda low temperature NO_(X) conversion peak below 200° C.; no such peak isevident in the activity of catalyst member C-3.

While the invention has been described in detail with reference toparticular embodiments thereof, it will be apparent that upon a readingand understanding of the foregoing, numerous alterations to thedescribed embodiments will occur to those skilled in the art and it isintended to include such alterations within the scope of the appendedclaims.

What is claimed is:
 1. A method for making a catalytic material usefulfor the reduction of NO_(X) comprising incorporating a catalyticallyeffective amount of a catalytic species into a template-bearingmolecular sieve material, and then calthe molecular sieve material,whereby the template is removed from the molecular sieve material afterthe catalytic species is incorporated therein.
 2. The method of claim 1wherein the molecular sieve material comprises an acid-form zeolitematerial selected from the group consisting of ZSM-5 and Beta-zeolite.3. A method for making a catalytic material useful for the reduction ofNO_(X) comprising depositing a blocking agent onto a molecular sievematerial, incorporating a catalytically effective amount of a cataltyicspecies into the molecular sieve material having the blocking agentthereon, and then calcining the molecular sieve material, whereby theblocking agent is removed from the molecular sieve material after thecatalytic species is incorporated therein.
 4. The method of claim 3wherein the molecular sieve material is selected from the groupconsisting of ZSM-5, Y-zeolite, mordenite, Beta-zeolite, omega-zeolite,rho-zeolite, borosilicates and iron silicates.
 5. The method of claim 2or claim 4 wherein the catalytic species comprises a platinum groupmetal.
 6. The method of claim 1 or claim 3 wherein the molecular sievematerial comprises ZSM-5, the method comprising incorporating platinuminto the ZSM-5 in an amount of less than about 2% by weight ZSM-5 plusplatinum.